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New Techniques to Sound the Composition of the Lower Stratosphere and Troposphere from Space

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New Techniques to Sound the Composition of the Lower Stratosphere and Troposphere from Space New techniques to sound the composition of the lower stratosphere and troposphere from space B.J.Kerridge,R. Siddans,J. Reburn,V. JayandB. Latter CCLRC, Rutherford Appleton Laboratory, Chilton, Oxfordshire, OX11 0QX [email protected] ABSTRACT This paper outlines three new techniques to sound the composition of the lower stratosphere and troposphere from space which are under development by the RAL Remote Sensing Group within NERC’s Data Assimilation Research Cen- tre. The focus is on retrieval of ozone, water vapour and other trace gases whose distributions are directly or indirectly important to climate. The first technique is to combine information from Hartley band absorption with temperature- dependentdifferentialabsorption in the Huggins bands to retrieve height-resolvedozone profiles spanning the troposphere and stratosphere from observations of backscattered solar uv radiation by ESA’s Global Ozone Monitoring Experiment (GOME). To illustrate the performance of the RAL scheme, ozone distributions and linear diagnostics are presented along with comparisons with ozonesondes and other space observations. The second technique is to exploit the synergy between limb- and nadir- observations of a given airmass to retrieve profiles spanning the troposphere and stratosphere which improve on those attainable by either geometry individually. Results are presented from the first application of this technique in the retrieval domain to co-located measurements from Envisat MIPAS and ERS-2 GOME. The third technique is to retrieve horizontal as well as vertical structure (ie 2-D fields) in the upper troposphere and lower strato- sphere through tomographic limb-sounding. This is illustrated with linear diagnostics and constituent fields retrieved by the RAL scheme in non-linear, iterative simulations for (a) a mm-wave limb-sounder (MASTER) and (b) Envisat MIPAS in its UTLS Special Mode (S6). Finally, the current status and plans for future work in these areas are summarised. 1 Background The distributions of certain trace gases in the troposphere and stratosphere can influence climate either directly (eg H2O and O3) through their radiative properties or indirectly (eg tropospheric CO or stratospheric ClO) through chemical interactions with radiatively active gases. The tropospheric distributions of trace gases such as O3, CO and CH4 reflect surface emission fluxes as well as chemical and physical processes occurring within the atmosphere. Tracer distributions can supplement the information on dynamics which can be derived from temperature fields. For these and other reasons, considerable attention is now being paid to the assimilation by global models of satellite observations of H2O, O3 and other trace gases. The aim of the RAL Remote-Sensing Group within the NERC Data Assimilation Research Centre is to produce novel global data-sets for assimilation by applying new techniques to sound the composition of the lower stratosphere and troposphere from space. In the following sections of this paper, three such techniques are briefly described. 73 KERRIDGE, B.J. ET AL:NEW TECHNIQUES TO SOUND THE LOWER STRATOSPHERE & TROPOSPHERE 2 Ozone Profiles from GOME A scheme has been developed to retrieve height-resolved ozone profiles from measurements of solar uv backscattered radiation made by ESA’s Global Ozone Monitoring Experiment (GOME) on ERS-2. A novel feature of the RAL scheme is to exploit temperature dependent differential absorption structure in the Hug- gins’ bands to extend to lower altitudes the information on stratospheric ozone which can be retrieved from the Hartley band. The strong, wavelength-dependent opacity of ozone in the Hartley band (260-310nm) has long been exploited by NASA’s ”BUV” type instruments and offers useful, height-resolved information down to the altitude of peak ozone concentration. In the RAL scheme, this is used as a priori information for a second step employing the Huggins bands. Two pre-requisites for this are: (1) contiguous spectral coverage at 0.2nm sampling afforded by GOME (cf measurements in discrete spectral bands of 1nm width by the ”BUV” type instruments) and (2) fitting precision in the 325-335nm interval of <0.1% RMS (cf 1% in the Hartley band). To achieve a fitting precision of <0.1% RMS, a number of instrumental and geophysical variables additional to ozone must be accounted for. The value of adding the Huggins bands is illustrated in 1 which compares aver- aging kernels for the Hartley band only with those for the combined Hartley and Huggins bands for a retrieval level spacing of 6km and an a priori uncertainty of 100% at all levels. Averaging kernels for the Hartley band are seen to have well-defined peaks only in the stratosphere, whereas those for the composite retrieval are seen to be well-behaved also in the troposphere. An independent intercomparison of averaging kernels has recently been performed by BIRA-IASB for the ESA Working Group on GOME Ozone Profile Retrieval (V.Soebijanta, pri.comm.). Figures 2 and 3 present the six most significant eigenvectors of the averaging kernel matrices for a typical ozone profile retrieval by the RAL scheme and by the IFE and KNMI schemes. The 5th and 6th eigenvectors of the RAL matrix are seen to have significant contributions from the lower troposphere, unlike the other two schemes. Profiles retrieved by the RAL scheme have been validated against a large ensemble of ozonesondes and also concurrent satellite observations. A number of sub-sets of data have been processed from the period 1995-9 (www.badc.rl.ac.uk) and a seasonal climatology of height-resolved ozone which spans the troposphere as well as the stratosphere has recently been produced [Siddans (2003)]. Figure 4 displays seasonal mean maps of ozone retrieved in the lower troposphere from ground-pixels in which cloud fraction has been estimated to be <0.05 Elevated ozone concentrations over south-east Asia and central America during northern hemisphere spring are associated with biomass burning. Over the USA, the north Atlantic and Europe, ozone concentra- tions are highest in summer because of photochemical production from primary pollutants and the prevailing westerlies. Anomalously high concentrations near the coast of Antarctica in southern spring are retrieval artifacts associated with ozone hole occurrence in the overlying stratosphere which are under investigation. Absence of data in a region extending over much of South America and the neigbouring Atlantic is because detector dark-current fluctuations caused by particle impact in the South Atlantic Anomaly prevent the use of the Hartley band. From 1999 onwards, degradation of the GOME scan-mirror surface has caused a serious reduction in its uv reflectivity. This reduction in reflectivity has been found to depend strongly on time, wavelength and view angle and exhibits the properties of a Fabry-Perot etalon of steadily increasing thickness. Because this reduction is greater in the direct-sun view than in the nadir-view, sun-normalised radiances actually appear to have increased (by up to 80%) after 1998. An empirical correction scheme has been devised by fitting a 2-D Legendre polynomial, of 4th order in wavelength and 14th order in time, to the fractional difference between sun-normalised spectra between 260 and 310nm measured by GOME and predicted by a radiative transfer model using the Fortuin and Kelder climatology [Fortuin and Kelder (1998)] (figure 5). An initial assessment through comparisons with ozonesondes in 1998 and summer 2002 (figure 6) indicates that the correction scheme is functioning adequately. The standard deviation of retrieved profiles with respect to the sondes is comparable or even smaller in 2002. The bias is also comparable in 2002, except in the 12-16km 74 KERRIDGE, B.J. ET AL:NEW TECHNIQUES TO SOUND THE LOWER STRATOSPHERE & TROPOSPHERE layer where it has increased from +10% to +20%. The RAL scheme has a negative bias of several 10’s% in the 0-6km layer. Simulations have shown that inadequate knowledge of slit-function shape in the Huggins bands can give rise to a bias of this magnitude [Kerridge et al. (2002),Siddans (2003)]. The impact of the degradation correction on data quality in the stratosphere has also been gauged through an assimilation exercise (M.Juckes, pri.comm.). Figure 6 shows scatter plots of assimilated GOME data vs MI- PAS, SAGE-III, HALOE and GOME itself on the 650K potential temperature surface (30hPa) for September 2002. The assimilated GOME data are biased with respect to the SAGE-III and HALOE solar occultation ob- 1 servations by +0.24 and +0.55ppmv, respectively, ie <10% . Initial indications from comparisons with ozonesondes and other satellite instruments are therefore that the degradation correction is performing sufficiently well to justify processing data from 1999-2003 as well as 1995-1998, ie the full (8-year) mission, to examine interannual variability in the troposphere and stratosphere. Figure 1: Composite averaging kernels for an O3 retrieval from GOME Bands 1 and 2 adopting, for clarity, a uniform retrieval level spacing of 6km and an a priori uncertainty of 100% at each level. In the right-hand panel, a noise floor of 1% has been imposed, which is appropriate for Band 1 (Hartley band) but effectively excludes information from Band 2 (Huggins bands). In the left hand panel, a noise floor of 0.1% is imposed, which is appropriate for Band 2, showing the benefit in the troposphere of adding the Huggins bands. 1MIPAS data exhibited anomalous variability, especially at high southern latitudes, in the version used here, which has been reduced in subsequent versions 75 KERRIDGE, B.J. ET AL:NEW TECHNIQUES TO SOUND THE LOWER STRATOSPHERE & TROPOSPHERE Figure 2: The six most prominent eigenvectors of the averaging kernel matrix for a typical O3 retrieval (13th Jan 1997 at 10:41:12; 50.85N 3.85E) by the RAL scheme, as calculated by V.Soebijanta (BIRA-IASB) for the ESA GOME O3 Profile Working Group.
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